1. Field of the Invention
The present invention relates to a disk producing method for a photo film cassette. More particularly, the present invention relates to a disk producing method for a photo film cassette with high precision in size.
2. Description Related to the Prior Art
There is a known photo film cassette in which a leader of photo film is pre-contained in a cassette shell, and rotation of a spool causes the leader to advance through a photo film port to the outside, as disclosed in U.S. Pat. Nos. 4,834,306 and 4,832,275, and U.S. Pat. Nos. 5,271,577 and 5,407,146 (both corresponding to JP-A 3-37645). It is necessary in the cassette to transmit rotation of the spool to a roll of the photo film. The spool has a spool core and disks secured to ends of the spool core. Each of the disks is formed in a shallow cup shape, and covers the outermost turn of the photo film at one end of the roll, to avoid loosening of the roll in the cassette, in view of reliable transmission of the rotation of the spool.
When the disks are pushed and deformed by edges of the photo film, the photo film is released from being covered by the disks. In view of easiness in the deformation, the disks are thinly formed from resilient resin or the like. There are known methods of producing disks U.S. Pat. No. 5,211,348 (corresponding to JP-A 4-251841 and 5-119436), which disclose a vacuum forming method in which a web of thermoplastic synthetic resin is unrolled, partially heated and softened, and heated portions are sucked through a mold by a vacuum pump. This document also discloses use of an air-pressure forming or thermal molding press to form an uneven shape with recesses or protrusions. JP-A 6-67360 and 6-242552 disclose a process in which a vacuum forming method forms a plurality of recessed and protruded shapes for disks on a web of thermoplastic synthetic resin one at a time, and disks are punched out of the web at each of the disk-like portions.
Thermoplastic synthetic resin, to be used for the material of the disks, requires resistance to high temperature as high as 80.degree. C., resistance to fatigue from deformation and to fatigue from flexure when formed thinly, and low surface abrasion. Examples of the resin meeting those requirements are modified polyphenylene ether (PPE) resin film, polycarbonate film, polyolefin film and the like. The thermoplastic synthetic resin of those kinds is relatively expensive. To raise a proportion of the product number to an amount of the material, disk-like portions are formed in an extremely wide web with an arrangement of small intervals between them.
To heat thermoplastic synthetic resin as such, operation of the heating must be effected in consideration of the high softening point. Examples of heating methods include a heating plate method, a radiation heating method, and a stoving method. In the heating plate method, the web is contacted on a heating plate or hot plate. In the radiation heating method, the web is moved past a heater or an infrared light source and heated by radiation heat therefrom. In the stoving method, the web is moved past a heated space.
The radiation heating method and the stoving method have shortcomings of considerable irregularity in temperature, because those methods fail to heat a large area at a regular temperature. There occur web portions remaining unsucked in the vacuum forming. Disks with low quality will be produced from the unsucked portions as a result of using those methods.
The heating plate method is capable of heating the web with the highest uniformity among all of the above-mentioned heating methods. A problem remains: when the web is contacted on the heating plate in partial fashion, accidentally without regularity, only a contacted portion is extended. If a mold for the vacuum/air-pressure forming is used for pressing the web against the heating plate, the partially heated extended portion can not resist remaining in a wrinkled form. On the other hand, if only a small portion of the web is away from the heating plate during contact of almost the whole of the web on the heating plate, the small portion is not softened and not sucked in the vacuum forming. The unsucked small portion inevitably results in an unacceptable disk.